The present invention is related to three state logic circuits and, more particularly, to an improved power up circuit for use therewith that ensures that the output of the logic circuit is in a high impedance state during power up of the logic circuit.
In a three state logic circuit in which one of the output states is a high impedance state it is desirable to include circuitry operating in conjunction with the output enable portion thereof to keep the output of the logic circuit in a disable state during power-up of the supply voltage. Prior art logic circuits can be found which include power up disabling circuits to which the present invention is directed. However, some, if not all, such prior art circuits are limited in their ability to to deliver sufficient current drive to maintain the output of the logic circuit in a disabled state during power up of the circuit. This is especially true if the logic circuit includes a plurality of data/output stages controlled by a single enable/disable circuitry.
Hence, a need exists for an improved power up circuitry for disabling the outputs of a logic circuit whereby the logic outputs are maintained in high impedance states during power up of the voltage supply and which using minimal power during normal operation of the logic circuit.